1. Field of the Invention
The present invention relates to an ultrasonic observation apparatus for observing tissues of a specimen using ultrasonic waves, an operation method of the ultrasonic observation apparatus, and a computer readable recording medium.
2. Description of the Related Art
As an examination technique for breast cancer and the like using ultrasonic waves, a technique called ultrasonic elastography has been known (for example, see Japanese Laid-open Patent Publication No. 2004-135929). Ultrasonic elastography is a technique utilizing the fact that stiffness of cancer or a tumor tissue inside a living body differs depending on the progress state of a disease and each living body. In this technique, in a state where an examined portion is pressed from the outside, a quantity of distortion and a degree of elasticity of the examined portion are measured using ultrasonic waves, and the measurement result is displayed in an image as a tomographic image.
As an ultrasonic wave is attenuated in a living body, it is important to know the quantity of attenuation or the attenuation correction coefficient when an examination is conducted. As a technique of correcting attenuation caused by propagation of an ultrasonic wave, STC (Sensitivity Time Control) correction which changes an amplification factor according to a propagation distance has been known (for example, see Japanese Laid-open Patent Publication No. 8-131435). This technique is to change the amplification factor of a signal according to a reception depth, and is not able to correct attenuation depending on a frequency. For this reason, in an area having a large reception depth and distant from the ultrasonic transducer, a high-frequency component is attenuated and the spatial resolution is reduced.
Meanwhile, a technique for performing correction depending on the frequency of a signal is also known (for example, see Japanese Laid-open Patent Publication No. 9-173334). In this technique, after performing fast Fourier transform (FFT) on a received signal, attenuation correction is performed so as to conform with a given frequency spectrum, and then inverse fast Fourier transform (IFFT) is performed.
Further, there is also a technique in which, with respect to a signal on which FFT has been performed, an inverse number is obtained for each frequency, and a spectrum, which increases or decreases in an inverse pattern of that of a measurement signal, is obtained as a spectrum of a correction coefficient, and after multiplying the correction coefficient with the measured frequency spectrum of the signal, IFFT is performed (for example, see Japanese Laid-open Patent Publication No. 10-33529).